1. Field of the Invention
The present invention relates to a method of manufacturing a resin filled board and a method of manufacturing a multi-layered printed wiring board using the resin filled board. More specifically, the present invention relates to a method of manufacturing a resin filled board by selectively filling a resin in throughholes of the board through printing, and a method of manufacturing a multi-layered printed wiring board, i.e., a so-called multi-layered build-up wiring board by building up an insulating resin and a conductive layer on core board which is the resin filled board manufactured by the method.
2. Description of the Related Art
The manufacturing of a multi-layered build-up wiring board involves previously filling throughholes of a core board with a resin to planarize the surface of the core board before the core board is formed with a build-up layer made of an insulating resin. This is because the formation of the build-up layer without planarization would cause the build-up layer to fall into the throughholes of the board to lose the planarity, thereby failing to correctly form a resist on the build-up layer and openings through laser processing in subsequent steps, with the result that a defective multi-layered build-up wiring board is produced.
Referring now to FIGS. 1a to 1e, description will be made on a method of manufacturing a resin filled board (core board) for use in a conventional build-up multi-layered wiring board and the like.
First, as illustrated in FIG. 1a, hole 2 is formed through insulating resin board 1 having copper foils adhered on both sides thereof, followed by formation of conductive layers 3 on the surfaces of board 1 and on the wall surface of hole 2 through plating such as copper plating. It should be noted that the underlying copper foils on the surfaces of board 1 are omitted in the figures. A hole formed between top and bottom conductive layers 3 is referred to as “throughhole” 4.
Next, the surfaces of conductive layers 3 are roughened to form rough surfaces 5 as illustrated in FIG. 1b, and then filling resin 6 is filled in throughholes 4 as illustrated in FIG. 1c. Generally, the filling resin is a mixture mainly composed of an epoxy resin and an inorganic filler. Filling resin 6 may include an accelerator, a reactive diluent, a surface active agent, and a viscosity modifier for improving the workability and yield factor of the filling. The viscosity of filling resin 6 is adjusted approximately in a range of 10 to 100 Pa·s. Filling resin 6 is filled in throughholes 4 by printing it at positions corresponding to throughholes 4 using a screen plate or a metal mask which is formed with dots extending therethrough and having a diameter slightly larger than throughhole 4.
Next, filling resin 6 is thermoset at temperatures ranging from 130 to 180° C. FIG. 1d illustrates filling resin 6 after it is cured. Finally, as illustrated in FIG. 1e, the surfaces of board 1 is polished using a belt sander or a buff to scrape off portions of filling resin 6 swollen from the surfaces of board 1 for planarization, thereby completing the core board.
The resin filling step places importance on perfect planarization of the surface of the core board, and high adhesivity of filling resin 6 with the surface of conductive layer 3 on the inner wall of each throughhole 4. For improving the adhesivity of filling resin 6 with the surface of conductive layer 3 on the inner wall of each throughhole 4, the conventional manufacturing method employs the roughening of the surface of conductive layer 3 including the inner wall of throughhole 4 before filling resin 6 is filled in throughholes 4, as illustrated in FIG. 1b. This roughening approach forms miniature ruggedness on the surfaces of the conductive layer on the inner wall of each throughhole 4 to increase the area of the surface which contacts filling resin 6 later filled in throughhole 4 to increase the adhesivity. Techniques describing such a roughening approach are disclosed in JP-A-10-13029, JP-A-10-4261, JP-A-2000-40863, and the like.
When the surfaces of conductive layers 3 on board 1 are roughened before filling resin 6 is filled in throughhole 4, filling resin 6 is filled along the miniature ruggedness on the surface of conductive layer 3 on the inner wall of each throughhole 4 after filling resin 6 is thermoset, so that the adhesion interface is expanded between the surface of conductive layer 3 on the inner wall of each throughhole 4 and filling resin 6 to advantageously improve the adhesivity.
However, since the roughening is conducted over the entire copper layer exposing on the surfaces of board 1, the surfaces of conductive layers 3 on the top and bottom of board 1 as well as on the inner wall of each throughhole 4 are affected by the roughening to result in a rough surface having the miniature ruggedness.
Filling resin 6 printed after the roughening step once softens in the process of naturally increased temperature, when it is thermoset, so that its viscosity becomes lower. In this event, filling resin 6 with a reduced viscosity, particularly, the epoxy resin component which includes no inorganic filler, exudes between the miniature ruggedness formed on the surface of conductive layer 3 on each side of board 1 by the action of osmotic pressure. As a result, when throughholes 4 are arranged at a narrow pitch, for example, 0.5 mm or less, portions of filling resin 6 exuding from adjacent throughholes 4 come in contact and join together, as illustrated in FIG. 1d. More specifically, observation on filling resin 4 swollen over the entire core board reveals that the finished board includes a mixture of portions of filling resin 4 each individually protruding on throughholes 4 arranged at a wide pitch and portions of filling resin 6 joined together over a plurality of throughholes 4 arranged at a narrow pitch.
With the core board thus finished, the individual protrusions of filling resin 6 are readily scraped off in the next polishing step for scraping off swollen filling resin 6. While the individual protrusions of filling resin 6 are completely removed to achieve the planarization of the board, the portions of filling resin 6 joined together over a plurality of throughholes 4 cannot be fully scraped off, resulting in polishing residue 7 as illustrated in FIG. 1e. If the core board is additionally polished to completely remove polishing residue 7, the individual protrusions of filling resin 6 can be excessively scraped off to cause a defective core board which has the conductive layer of a thickness smaller than a prescribed value.
As illustrated in FIG. 1d, as filling resin 6 exudes around throughholes 4 and spreads thereover, printed filling resin 6 is less swollen. In addition, since overall filling resin 6 drops naturally through gravity due to the softening of the resin experienced in the middle of increasing temperature during the thermosetting, the resulting core board may have deep recesses in throughholes 4. Such deep recesses in throughholes 4 cause recess 6a on the surface of the core board after the polishing step, as illustrated in FIG. 1e, thereby failing to form a flat core board.
As appreciated from the foregoing, the roughening before filling resin 6 is filled in throughholes 4 advantageously improves the adhesivity of filling resin 6 with the surface of conductive layer 3 on the inner wall of each throughhole 4, whereas a board which partially includes throughholes at a narrow pitch would experience difficulties in uniformly filling all throughholes with the filling resin due to the roughening and have recesses in the throughholes to damage the planarity of the core board. The damaged planarity on the surface of the core board would cause air voids to be trapped on the interface between the core board and build-up resin layer, and reduce the planarity on the surface of the build-up resin layer.
The foregoing problem would impede the formation of a resist by photography, and the formation of correct openings through the resin by laser processing to reduce the yield factor. The air voids trapped in the interface between the core board and build-up resin layer also reduce the adhesivity of the build-up resin layer with the core board. The insufficient adhesivity would cause cracking within the multi-layered build-up wiring board, starting from a throughhole of the resin filled board which is the core board. In the worst case, the cracking would result in a broken conductive path on the build-up layer.